In such a mounting, considerable axial, radial and tilting forces must be absorbed by suitable bearings, usually a combination of journal and thrust bearings using rotary bodies such as rollers. Generally, the load is mounted on a platform resting on an annular carrier which is centered on the axis of rotation and is supported via the aforementioned journal and thrust bearings on a stationary annular member secured to the base. The two coaxial annular supports provide a free space around the axis in which the means for rotating the platform, such as a motor-driven shaft, can be accommodated.
A drawback of this conventional type of mounting is that the surfaces contacted by the bearing rollers are subject to considerable wear under load and thus require frequent replacement. In order to reduce the down-time caused by these repairs, it has already been proposed to provide a rotatable carrier with an ancillary set of bearing elements which are normally inactive but can be made effective to support all or part of the load when the main bearing elements encounter too much resistance due to excessive wear. Even in their inactive condition, however, the ancillary bearing elements are set in rotation by the associated contact surfaces and may therefore produce grooves in these surfaces even before a substantial part of the load is transferred to them.